1. Field of the Invention
The present invention relates in general to a two-mirror system for reshaping non-circular beams of optical pumping radiation so as to provide smaller diameter absorption volumes in an optically pumped laser gain medium.
2. DESCRIPTION OF THE PRIOR ART
In a diode-end-pumped laser, light from a laser diode or laser diode array is absorbed by the active atoms of a solid state laser gain material. In order to maximize the gain and minimize the size of the diode-pumped laser, it is desirable to minimize the volume in which the absorption of the diode light takes place.
To minimize the absorption volume, it is necessary to minimize the average cross-sectional area of the laser diode beam over the distance within which absorption takes place in the laser gain material. The pump beam should come to a focus at the entrance to the solid-state laser gain material and remain as small as possible throughout the absorption length.
Single-narrow stripe laser diodes, such as those commercially available from Toshiba, emit light from a very small aperture. Because the aperture height is small, the vertical divergence of the light leaving the aperture is large. For diode pumping of Nd:YAG, or of any other solid-state laser gain material with absorption lengths exceeding a few hundred microns, the pump beam divergence is so large that it requires some re-shaping for use of the beam. In a typical example of a solid state laser gain medium pumped by a single-narrow stripe laser diode, it is desired to absorb the light in a volume of length one millimeter and diameter 0.08 millimeters. The diode pumping beam has a 30 degree vertical divergence and diverges after one mm to a 0.5 mm diameter. It is conventional to use a SELFOC (refractive index) lens to magnify by a factor of six the beam emitted from the diode. This magnification decreases the divergence of the beam by a factor of six, so that after propagating one mm in the laser gain material, the beam only diverges to about 0.08 mm in diameter. Such a single-narrow stripe diode in the horizontal direction has an emitting width of five microns and a divergence angle of 10 degrees. Thus, after magnification by six, its horizontal divergence is very small. The size of the focus spot is six times five microns or 0.03 mm, still well within the goal of 0.08 mm.
Referring now to FIG. 1, there is shown a prior art short stripe laser diode 11S pumped laser employing a focusing lens 12.times.1 with a magnification of one. More particularly, optical pumping radiation emanating from the output aperture of laser diode 11 is gathered by spherical lens 12.times.1 and focused into the mode volume 13 of a laser gain material such as Nd:YAG 14. In this case, laser diode llS is a single-narrow-stripe laser diode which has a beam output aperture resembling a point source. This point source is imaged on the input face of the laser crystal 14. The laser pumping radiation is widely divergent from the output aperture of the laser diode llS and, therefore, as focused with a magnification of only one, it rapidly diverges and is not confined to the desired mode volume 13 such as a cylindrical absorption region 100 microns in diameter and 1,000 microns in length. As a result, pumping radiation is not properly matched to the desired mode volume 13, and inefficient and nonoptimum pumping of the laser crystal 14 results.
Referring now to FIG. 2, there is shown an improved version of the prior art of FIG. 1 wherein the magnifying lens 12.times.6 magnification is increased to a factor of, for example, six, which serves to reduce the divergence of a focused image in the mode volume 13 correspondingly by a factor of six. As a result, essentially all of the optical pumping radiation is absorbed in mode volume 13 of the laser crystal 14 and optimum beam-pumping conditions are obtained when pumping with a single-narrow stripe diode llS.
While single-narrow stripe laser diode llS beams typically require no special shaping for pumping small absorption volumes 13, they have the disadvantage that they are limited in the amount of pumping beam output power. On the other hand, wide-stripe diodes and diode arrays provide substantially greater optical beam pumping power but they have emitting areas which are relatively large in the horizontal dimension. For example, wide-stripe diodes such as those commercially available from Sony, or laser diode arrays such as those commercially available from Spectra Diode Laboratories, have emitting aperture widths of up to 200 microns. The horizontal width at the aperture of the pumping diode already exceeds the desired width of the absorption volume 13 in the laser gain material.
Referring now to FIGS. 3 and 4, there is seen the problem that arises when the single-narrow-stripe laser diode llS is replaced by a single-wide stripe or laser diode array 11L. In this example, the output aperture of the laser diode pump as shown in FIG. 3 is characterized as a non-circular or elongated aperture beam having a short height and a less narrow width of approximately 200 micrometers. When this source 11L is magnified by a magnifying lens 12.times.6, the horizontal width of the source 11L is imaged on the mode volume 13 greatly exceeds the width of the mode volume, i.e., as a width of approximately 1,200 micrometers as shown in FIG. 4 while the desired mode volume width is only 100 micrometers. As a result the pumped mode volume is not well matched to the desired mode volume 13, resulting in non-optimum pumping conditions and a substantial loss of efficiency. Because of the large width of these higher power diode arrays and the wide-stripe diodes 11L, it is not desirable to magnify their horizontal widths. The aperture heights and the angles of divergence for such higher power diodes 11L are about the same as for a single-narrow stripe laser diode. The larger vertical divergence angle means that it is still desirable to magnify the beam in the vertical direction. Thus, for a wide-stripe type diode, or a laser diode array, it is desired to magnify the vertical dimension by a factor of three to ten while leaving the horizontal dimension nearly unchanged or in fact demagnified.
Thus, it is desired to build an imaging system for shaping of the non-circular optical pumping beam from a wide-stripe diode or a diode array 11L which has no magnification in the horizontal direction while having magnification by a factor of three or more in the vertical direction.
Diode lasers also have a problem of astigmatism. This is equivalent to light diverging in the vertical plane being emitted from a different point than light diverging in the horizontal plane. If a diode beam is astigmatic, then it is not possible to focus its light to a point using only a spherical lens. The light will come to a line focus at one distance, and another line focus rotated by 90 degrees at another distance. A single tight focus is much preferred. Therefore, it is desired to obtain an optical beam-shaping or imaging system to eliminate the effects of astigmatism by independently positioning the foci in the horizontal and vertical directions. In this manner, it will be possible to force an overlap of the horizontal and vertical image depths of focus.
Two techniques for laser diode beam-shaping have been proposed. One conventional technique uses prisms and the other uses cylindrical lenses. Both of these techniques take advantage of the fact that prisms or cylindrical lenses differently affect light rays diverging in one plane as compared to light rays diverging in the orthogonal plane.
Both techniques have the disadvantage that the optical components and the distances needed to allow the components to have their desired effect, are large. In diode-pumping, it is desired to minimize size. In addition, cylindrical lenses are fairly expensive. In the prism techniques, additional lenses are needed both before and after the prisms.